Motor-driven screw driver with automatic stopping means

ABSTRACT

An apparatus for driving-in and out a screw which comprises in combination an electrically operated screw driver and electrical means for automatically controlling torque of the screw shank upon the final stage of screw fastening operation. The automatic control of torque is accomplished by a switching circuit for detecting a load current flowing through the motor with a value of overload current larger than a predetermined value and a predetermined duration thereat so as to interrupt supplying current to the motor and subsequently restoring energization of the motor after lapse of a predetermined time interval.

United States Patent Totsu et al.

1 MOTOR-DRIVEN SCREW DRIVER WITH AUTOMATIC STOPPING MEANS [76] Inventors: Katsuyuki Totsu, 4-7, 3 Chome Oshiage, Sumido-ku; Toshimasa Shimizu, 21-14, 3 Chome Nakajujo Kita-ku, both of Tokyo, Japan [22] Filed: May 21, 1973 [21] Appl. No.: 362,242

[30] Foreign Application Priority Data May 22. 1972 Japan 47-50509 Mar. 20, 1973 Japan 48-32471 [52] US. Cl 318/434, 317/22, 318/476 [51] Int. Cl. H02h 7/085 [58] Field of Search 317/14 C, 13 R, 22, 33 R, 317/36 TD, 151, 157; 318/127, 122, 134, 434, 474, 476; 323/22 Z; 307/127 [56] References Cited UNITED STATES PATENTS 3,246,170 4/1966 Olshan 323/22 Z 1 Oct. 29, 1974 3,395,324 7/1968 Rager, .Ir. 307/127 X 3,555,357 l/1971 Gregson et al.... 317/22 X 3,644,789 2/1972 Simpson 317/36 TD 3,660,718 5/1972 Pinckaers 1 1 317/22 X 3,703,679 11/1972 Hieidt..........,. 317/36 TD 3,742,302 6/1973 Neill 317/22 X Primary ExaminerRobert K. Schaefer Assistant Examinerw. E. Duncanson, Jr.

[57] ABSTRACT An apparatus for driving-in and out a screw which comprises in combination an electrically operated screw driver and electrical means for automatically controlling torque of the screw shank upon the final stage of screw fastening operation. The automatic control of torque is accomplished by a switching circuit for detecting a load current flowing through the motor with a value of overload current larger than a predetermined value and a predetermined duration thereat so as to interrupt supplying current to the motor and subsequently restoring energization of the motor after lapse of a predetermined time interval.

5 Claims, 2 Drawing Figures MOTOR-DRIVEN SCREW DRIVER WITH AUTOMATIC STOPPING MEANS BACKGROUND OF THE INVENTION The present invention relates to an apparatus for driving-in and out a screw comprising, in combination. a screw driver driven by an electric motor and a device for controlling torque of the motor so as to automatically stop it dependent upon load condition imposed on the screw shank of the screw driver.

Heretofore. such motor-driven screw drivers of various types for fastening screws have been proposed which are incorporated with an electric motor and driven electrically thereby. Generally, in motor-driven screw drivers, substantial part of the fastening process is carried out in a short period through rotation of the driver shank under high load condition in engagement with the screw slot. and the final fastening process is achieved by the correspondingly increased torque of the driver shank due to the frictional resistance between the screw and a screw receiving member.

Upon final fastening operation of a screw by such a motor-driven screw driver, if the driving torque is excessively high. the screw threads, the slot on the screw head or even the screw head itself may be destroyed, or any screw receiving member may be damaged and. moreover. considerable delay of switching-off the motor after fastening has been completed will cause overheating of the motor. If the driving torque is weak, sufficient fastening is not accomplished so that the screw will be caused to gradually detach from the screw receiving member. Under such situations it is very diffcult to stop the driving motion of the screw at predetermined limit while supplying suitable force for fastening.

Therefore. in motor-driven screw drives of the prior art it has been proposed to provide an idling mechanism which operates when a predetermined reaction is imparted to the driver shank. However, such the idling mechanism has such disadvantages and inconveniencies that it makes the screw driver assembly complicated; degradation infunctions of the associated parts due to wear caused by their repetive sliding operations is unavoidable; and its manufacturing takes longer time. whereby it is unsatisfactory for practical use.

Therefore, upon operation of the conventional motor-driven screw driver, suitable fastening of the screw is effected by turning'off an activation switch of the driver through the operator's sensitivity as human being or through the operator's prediction of instant at which fastening operation will come to an end so that it is very difficult to always realize constant fastening which requires considerably skill.

After extensive study of these problems, the inventors have traced the fact that when the driver shank is prevented from its rotation by some external factors such as friction the current flowing through the rotor of the motor increases drastically, and consequently found that an automatic torque control of the motordriven screw driver can be acheived by providing a controllable source of power supply so constructed that when the load current flowing through the motor for driving the driver shank has reached an overcurrent caused by preventing the rotation of the motor resulting from completion of fastening. energization of the motor from said power source is interrupted by electrically detecting said state to automatically stop the rotation of the motor-driven screw driver. As a result, it was recognized that an operation of the motor-driven screw driver can be simplified and also an efficiency in working can be enhanced.

In this case, a transistor switching circuit is provided in the power circuit formed on the voltage feed line connected to the motor section of the motondriven screw driven, so that the switching circuit effects switching action to interrupt a current supply circuit when the load current which increases steeply under reaction against rotation of the motor reaches a predetennined value at the final stage of fastening operation by means of the driver shank. While upon starting the motor an excessive load current beyond its rated current flows momentarily and thus the switching circuit operates immediately thereafter to switch-off the power circuit. In general. however. since such an excessive load current also flows instantaneously upon starting of the motor. the switching circuit tends to cut off the power circuit upon starting, thereby rendering an actual driving of the motor-driven screw driver impossible. Thus, it was recognized that such undesired condition can be eliminated by a parallel combination of an overvoltage relay and a capacitor together with a resistor in series with said capacitor. the time constant of which capacitor and resistor is chosen so that the overvoltage relay does not respond to momentary overload currents such as starting current because the capacitor is not charged sufficiently for activation of said relay.

Moreover. it was found that the screw driver assembly is suitable for repetitive fastening work as effected in the mass production line if the power circuit is so constructed that its current feed circuit can return to its active state afier lapse of the predetermined time interval.

In addition. it was found in case of a screw driver assembly driven by a dc. motor contained therein that a motor-driven screw driver which enables to drive-out or detach a screw effectively can be obtained providing a polarity changing switch at a junction with the power circuit so that one of its switching contacts for reversing the motor is connected directly to a dc. voltage line without otherwise altering said switching circuit.

SUMMARY OF THE lNVENTlON Accordingly, a general purpose of the present invention is to provide an apparatus for driving-in and out a screw which comprises in combination an electrically operated screw driver and electrical means for auto matically controlling torque of the screw shank upon final stage of the screw fastening operation.

The principal object of the invention is to provide a screw driving apparatus having an automatic stopping means. which comprises in combination an electrically operated screw driver having a driver shank driven by an electric motor contained in the screw driver assembly and a switching circuit for detecting a load current flowing through the motor having both a value of overload current larger than a predetermined value and a predetermined duration thereat so as to interrupt supplying current to the motor and subsequently restoring energization of the motor after lapse of a predetermined time interval.

A further object of the invention is to provide a screw driving apparatus of the type mentioned above which is characterized by comprising a transformer, a rectifying means to drive a dc. voltage, and a switching circuit including an adjustable constant voltage generator circuit connected to an output of said rectifying means and a load resistor connected to an output lead of the dc. voltage so as to compare a voltage across said load resistor produced by current passing through the motor with a predetermined voltage set by said constant voltage circuit and moreover so as to interrupt energization of the motor by activating relays when the current through said load resistor changes to an excessive current beyond a predetermined value and sustains such excessive value during a predetermined priod and enable said relays to restore after a predetermined time having elapsed.

Another object of the invention is to provide a screwdriving apparatus of the type as described above, which is characterized by providing a polarity inverting switch between d.c. output leads and the drive motor, contacts of the polarity inverting switch for rotating the motor in the direction of driving-in the screw being connected via said switching circuit to an output of the rectifying means whereas contacts of the polarity inverting switch for reverse rotation of the motor being connected directly to the output of said rectifying means.

Still a further object of the invention is to provide a screw driving apparatus of the type as described above comprising an automatic stopping means for the motor upon fastening the screw, wherein a switch is connected in series with the capacitor parallel with the relay in the switching circuit for switching off the do voltage output lead so as to stop the switching action of the switching circuit.

Other objects and advantages of the invention will be apparent from the following disclosure taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a block diagram showing a basic construction of the apparatus according to the invention; and

FIG. 2 is a schematic diagram of an embodiment of the apparatus in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 a screw driving apparatus according to the invention is constructed in such a manner that a rectifying device 12 is connected to a commercially available power source E though a step-down transformer for deriving suitably lowered d.c. voltages and an output of the rectifying device 12 is connected to a switching circuit 14 which is main part of the invention, the output of the switching circuit 14 being connected to the input of a motor-driven scew driver 16.

Referring to FIG. 2, in the switching circuit 14 a resistor R, and a Zener diode are connected in series between output leads I80 and 18b of the rectifying device 12 and a capacitor C, is connected in parallel with the Zener diode ZD so that a constant voltage is produced at junction point A.

Across the Zener diode ZD a resistor R, and a variable resistor VR are connected in series so that a constant voltage can be derived adjustably from a sliding tap (p) of the variable resistor VR.

A load resistor R, is connected to one of the output leads 18b of the constant voltage generator circuit thus constructed. Upon energization of the motordriven screw driver 16 a load current I of the motor contained therein flows through the load resistor R, across which a voltage which is a function of the load current 1 appears. In order to compare the voltage across the load resistor R, with the constant voltage obtained at the sliding tap (p) of the variable resistor VR, a transistor TR, is connected in parallel with the load risistor R The transistor TR, has an emitter ((2,) connected to the sliding tap (p) of the variable resistor VR and a base (b,) connected via a suitable protection resistor R, to the end of the load resistor R, opposite to the variable resistor VR. A collector (c,) of the transistor TR, is connected via series combination of resistors R, and R to the output lead 180.

Junction point of the resistors R and R, is connected to a base (12,) of a second transistor TR,, the emitter (e,) of which is connected through a protection diode (D) to the output lead 18a and the collector (Cg) thereof is coupled to the output lead 18b through series combination of a resistor R and a relay RY, shunted by a capacitor C As a result, if the voltage at the base (b,) of the transistor TR, becomes higher than that at its emitter (e,), a current flows via collector (c,), and consequently the base (h of the second transistor TR, is forward-biased thereby rendering the latter conductive. That is to say, a load current caused upon rotation of the motor contained in the screw driver assembly 16 flows through the circuits constructed as mentioned above, and if the collector current of the transistor TR, exceeds a predetermined value as the load current increase, the transistor TR, is turned on to pass the current on its collector (c,). The capacitor C, is charged fully by this collector current within a predetermined time of period (for example 0.1 to 0.3 seconds dependent upon a time con stant determined by the resistor R, and a capacitor C,, a voltage enough to activate the relay RY, appears across the capacitor C Therefore, the relay RY, does not respond to any instantaneous overcurrent produced upon starting of the motor, and it is ensured that the relay RY, becomes operative only when the capacitor C, is changed fully by sustaining overcurrent caused upon the final stage of fastening work with the motordriven screw driver 16.

By this operation of the relay RY, its normally opened contact 20 is closed and hence a subrelay RY which is connected in series with the contact 20 be tween output leads 18a and 18b, is activated, so that its normally closed contact 22 which is connected to the output lead 18b including the load resistor R, and serves as the cut-off switch is made open, resulting in an interruption of the switching circuit 14.

When the switching circuit 14 is interrupted, the load current becomes zero and thus the voltage at base (12,) of the transistor TR, decreases below that at the emitter (e,), so that its collector current (control current) also reduces and consequently the transistor TR, is turned off. However, due to discharging of the capacitor C, through the relay RY, the latter is maintained in its activated condition until the voltage across the capacitor C, decreases below a predetermined activation voltage for energizing the relay RY, in consequence of which the switching circuit 14 is held in the open state. Accordingly, the time required for discharging of the capacitor C to a voltage thereacross lower than the predetermined activation'voltage of the relay RY, is set, for example to several seconds, so that maximum efficiency in operation of the motor driven screw driver which acts as load can be obtained.

When a motor-driven screw driver thus constructed is emloyed, fastening operation can be achieved for example, within several seconds for each member or product to be screwed these members being successively fed; in mass production system. Upon completion of a fastening operation the current supply to the motor-driven screw driver is interrupted automatically. thereby reducing shock upon stopping of the motor. Subsequently. the switching circuit is turned on again before the next member or product to be screwed arrives at the fastening station, and the current is applied to the motor so as to start rotation of the driver shank again, resulting in the next fastening work being ready. In addition, a polarity inverting switch 24 is connected between the dc. output leads 18a, 18b and the input of the screw drive assembly 16 so that polarity of the input voltage to the drive motor can be changed. In this case, the circuit connection is such that in the switch position for reverse rotation of the drive motor the input thereof is coupled directly't'o the rectifying device 12, thus by passing the load resistor R Therefore, upon driving the motor in rotation of reverse direction a higher torque than that produced upon forward roation of the motor is produced and this enables screws or nuts to be detached effectively. it should be noted that upon reverse rotation of the motor the protection of the circuit arrangement is carried out by a circuit breaker 26 provided between one output of the rectifying device 12 and the output lead 18a.

Moreover, in the above-mentioned switching circuit, a switch 28 is connected across the relay RY, in series with the capacitor C for activating the relay RY,, and this switch 28 is located at the screw driver assembly itself or other suitable places. When the switch 28 is made open, the capacitor C is not charged and consequently the relay RY, can be deactivated. Therefore, when two or more kinds of fastening work which require extremely different torques are to be effected alternately, desired fastening within the range of maximum torque produced by the screw driving apparatus can be achieved successively without interruption by suitable manipulation of the switch 28.

In the screw driving apparatus according to the invention, terminals of the primary winding of the transformer are connected to a commercially available power source E through a fuse 32 and a switch 30, respectively, and the secondary winding of the transformer I0 is provided with a plurality of taps so that desired various voltages can be derived through a movable contact arm 34 by engaging it with any of said taps. Moreover, a smoothing capacitor 36 is connected at the output of the rectifying device 12 between the leads I80 and 18b.

As seen from the foregoing, the screw driving apparatus of the invention enables screw fastening work to be carried out together with rotating speed and fastening force adapted to desired screws or nuts, and renders it possible that the current feed circuit for driving the motor is opened by a load current or a short-circuit current which flows for a time of period set arbitrarily at a constant torque of fastening so that the motor is stopped automatically. As a result, shock occuring at the completion of fastening work can be eliminated. Then, within a predetermined interval after stopping the motor said circuit restores its active state to drive the motor again, and thus the next screw fastening work becmes ready immediately.

Moreover, fastening work of various types can be ac complished by suitable interruption of operation of the switching circuit.

By employing the screw driving apparatus according to the invention not only the final portion of screw fastening can always be achieved with optimum torque, but also its operation does not need skill and there is no possibility that an overcurrent flows through the motor for long time, thereby providing various advantages such that life of the screw driver can be increased.

While the invention has been shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

We claim:

1. A screw driver apparatus comprising a motor driven screw driver, a selectively variable source of energy for energizing the motor driven screw driver with a prdetermined driving torque, a manually positionable electrical switch having at least two positions in one of which the motor driven screw driver is connected for energization by said source of energy through solely resistive loading means which produces a voltage drop the magnitude of which is related to the magnitude of the motor current flowing through the resistive loading means, and electircal circuit means including said resistive loading means and being operative in said one position of said electrical switch for (a) automatically terminating operation of the motor driven screw driver when the load current through said resistive loading means exceeds a predetermined magnitude for a predetermined time and (b) for resetting operation of the motor driven screw driver after a predetermined time subsequent to said termination of operation, said electrical switch in said other position thereof by-passing said circuit means and said loading means and being directly connected to said source of energy.

2. Apparatus according to claim 1, wherein said circuit means comprises first and second transistors, each having an emitter, a base and a collector, the emitter of said first transistor being connected to an adjustable level of said source of energy, said loading means comprising a resistor electrically connected between said emitter and the base of said first transistor which when rendered conductive during abnormal current flow through said loading means causes current to flow through its collector whereby to forward bias the base of said second transistor in relation conductive, the collector of said second transistor being electrically connected to a relay having a capacitor connected in shunt therewith, said capacitor delaying for a predetermined time energization of said relay upon conduction of said second transistor to delay, for said predetermined time, opening of a switch contact adapted for series connection with the motor of the motor driven screw driver, said capacitor also delaying for a predetermined time deenergization of said energized relay after termination of operation of said motor driven screw driver.

3. Apparatus according to claim 2, wherein said electrical switch has a pair of movable contacts movable for selective engagement with first and second fixed pairs of contacts, said movable contacts being connected to the motor of the motor driven screw driver, said first pair of contacts, through the intermediary of said series connected contact, being connected to said source of energy in series with said loading means, said second pair of contacts being connected to said source of energy for drive of the motor of the motor driven screw driver when said movable contacts engage said second pair of contacts, in a direction opposed to the direction of its drive when the movable contacts engage said first pair of contacts.

4. Apparatus according to claim 2, wherein a further switch is provided in series connection with said capacitor, said series connected switch and capacitor being in shunt connetion with said relay.

5. Apparatus according to claim 1, wherein said motor of said mtor driven screw drivr is a dc. motor, said selectively variable source of energy comprising a transformer having a primary winding connected across a source of alternating current and a secondary with multiple taps each of different voltage, a selector switch for selecting said taps and being connected to a voltage rectifier for converting said alternating source into a direct current source whose magnitude depends on the tap on the secondary winding selected by said selector switch, said do motor being energized by said direct current source.

F i k i 

1. A screw driver apparatus comprising a motor driven screw driver, a selectively variable source of energy for energizing the motor driven screw driver with a prdetermined driving torque, a manually positionable electrical switch having at least two positions in one of which the motor driven screw driver is connected for energization by said source of energy through solely resistive loading means which produces a voltage drop the magnitude of which is related to the magnitude of the motor current flowing through the resistive loading means, and electircal circuit means including said resistive loading means and being operative in said one position of said electrical switch for (a) automatically terminating operation of the motor driven screw driver when the load current through said resistive loading means exceeds a predetermined magnitude for a predetermined time and (b) for resetting operation of the motor driven screw driver after a predetermined time subsequent to said termination of operation, said electrical switch in said other position thereof by-passing said circuit means and said loading means and being directly connected to said source of energy.
 2. Apparatus according to claim 1, wherein said circuit means comprises first and second transistors, each having an emitter, a base and a collector, the emitter of said first transistor being connected to an adjustable level of said source of energy, said loading means comprising a resistor electrically connected between said emitter and the base of said first transistor which when rendered conductive during abnormal current flow through said loading means causes current to flow through its collector whereby to forward bias the base of said second transistor in relation conductive, the collector of said second transistor being electrically connected to a relay having a capacitor connected in shunt therewith, said capacitor delaying for a predetermined time energization of said relay upon conduction of said second transistor to delay, for said predetermined time, opening of a switch contact adapted for series connection with the motor of the motor driven screw driver, said capacitor also delaying for a predetermined time deenergization of said energized relay after termination of operation of said motor driven screw driver.
 3. Apparatus according to claim 2, wherein said electrical switch has a pair of movable contacts movable for selective engagement with first and second fixed pairs of contacts, said movable contacts being connected to the motor of the motor driven screw driver, said first pair of contacts, through the intermediary of said series connected contact, being connected to said source of energy in series with said loading means, said second pair of contacts being connected to said source of energy for drive of the motor of the motor driven screw driver when said movable contacts engage said second pair of contacts, in a direction opposed to the direction of its drive when the movable contacts engage said first pair of contacts.
 4. Apparatus according to claim 2, wherein a further switch is provided in series connection with said capacitor, said series connected switch and capacitor being in shunt connetion with said relay.
 5. Apparatus according to claim 1, wherein said motor of said mtor driven screw drivr is a d.c. motor, said selectively variable source of energy comprising a transformer having a primary winding connected across a source of alternating current and a secondary with multiple taps each of different voltage, a selector switch for selecting said taps and being connected to a voltage rectifier for converting said alternating source into a direct current source whose magnitude depends on the tap on the secondary winding selected by said selector switch, said d.c. motor being energized by said direct current source. 